The process by which coal is created through geological time is well known and well documented by numerous investigators. Also well known is the face that by-products of the coalification process include copious quantities of methane and lesser quantities of carbon dioxide, hydrogen and other gases. Virtually all coal seams when opened give off methane, with some coal beds containing as much as 600 standard cubic feet of methane trapped in each ton of coal. Coal may be generally described as a carbonaceous solid material that contains a considerable amount of pore space. Pore space readily visible to the naked eye is commonly called macropore space while that requiring magnification for observation is commonly called micropore space. Methane entrained in coal is located in the macropores (normally cracks and fissures) as free methane and in the micropores as methane adsorbed on to the surfaces of the coal.
Methane, being lighter than air, will freely escape from the macropores when coal is exposed as in an outcrop to the surface of the earth. Methane adsorbed in the micropores will also escape to a substantial degree when coal is exposed but the rate of desorbtion is generally quite slow, and many years of exposure may be required before the desorbtion release of methane is complete.
The coal miners of ancient times had little problem with methane because they were removing coal from outcrops and virtually all of the methane originally entrained in the coal had long since escaped to the atmosphere. When the coal was mined farther and farther underground and away from the outcrop, miners became very much aware of what they called firedamp (principally methane) and blackdamp (principally carbon dioxide) because of the hazardous nature of the gases.
Much research work was done through the years to identify and control the hazards associated with firedamp and blackdamp. Currently the most common control of these hazards is abundant ventilation of underground workings so that the gases emitting from the coal are diluted to safe levels in circulating air that is vented to the atmosphere. While this is an effective safety measure, valuable methane is wasted by dispersal into the atmosphere.
The U.S. Bureau of Mines has performed numerous detailed studies of methane emission from coal. In one recent study (Bureau of Mines Information Circular 8558, 1972) the Bureau reported one coal mine in West Virginia that had an emission of methane exceeding 12 million standard cubic feet per day. Those engaged in the petroleum industry can readily visualize this coal mine as a natural gas reservoir that would be the equivalent of a natural gas field containing a hundred or more commercial gas wells. The problem, of course, if the coal deposit is to be produced as a natural gas reservoir is to attain flow rates into the production wells at commercially acceptable levels.
Among the studies performed by the U.S. Bureau of Mines that have been widely published are Report of Investigations No. 7762 (1973) which deals with the composition of coal seam gas, an article in the American Gas Association Monthly (January 1974) which describes degasification of coal beds as a commercial source of pipeline gas, an article in the Oil and Gas Journal (June 16, 1975) which points to coal beds in the United States as a commercial source of natural gas, and an article in Coal Mining and Processing (October 1976) which reports on methane drainage from five horizontal holes into the Pittsburge seam at a site in West Virginia. In this last named report the Bureau discloses the wells to be 3.5 inches in diameter with average bore length of 1200 feet, with production during a three year period of 770 million standard cubic feet of methane. The Bureau expects these five wells to produce about 600,000 cubic feet per day of methane for 3 more years. The recovered methane is sold to a local natural gas utility which in turn delivers it to its customers as a product undistinguishable from natural gas of petroleum origin.
Draining methane from coal at commercially acceptable withdrawal rates requires large areas of the coal to be exposed. In a relatively thin seam such as the Pittsburgh seam, vertical holes into the seam are generally unrewarding in the commercial sense because of the small surface area exposed to drainage, hence the horizontal holes. Drilling horizontal holes from an outcrop into a coal seam also is generally unrewarding because so much of the methane originally trapped in the coal has already escaped to the atmosphere. The cost of sinking a shaft to an underground coal seam so that horizontal wells may be drilled generally exceeds the sales price of the methane expected to be recovered.
It is an object of the present invention to disclose methods wherein vertical wells are drilled into a coal seam with large areas of coal being exposed to drainage through the production wells. Other objects and advantages of the present invention will become apparent as the description proceeds.